Organic EL devices are self light-emitting devices which feature higher brightness and higher legibility than those of liquid crystal devices enabling vivid display to be realized, and have, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of Eastman Kodak Company have developed a device of a laminated structure comprising various kinds of materials to bear individual roles, and have put an organic EL device using organic materials into a practical use. The above organic EL device is constituted by laminating layers of a fluorescent body capable of transporting electrons and of an organic material capable of transporting holes. Because of this configuration, the organic EL device is adapted to inject positive charges and negative charges into the layer of the fluorescent body to perform light emission, thereby obtaining a high luminance of 1,000 cd/m2 or more at a voltage of 10V or less.
So far, many improvements have been made to put the organic EL device into practical use. For example, it is generally well known that high efficiency and durability can be achieved by an electroluminescence device having a laminated structure, in which the roles to be played by respective layers are further segmented, i.e., having an anode, a hole injection layer, a hole transport layer, a luminous layer, an electron transport layer, an electron injection layer, and a cathode on a substrate.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and study has been forwarded to utilize a phosphorescent luminous compound. Devices have, further, been developed utilizing the emission of light based on the thermally activated delayed fluorescence (TADF). In 2011, Adachi et al. of Kyushu University has realized an external quantum efficiency of 5.3% by using a device comprising a thermally activated delayed fluorescent material.
The luminous layer is, usually, prepared by doping a charge transporting compound called host material with a fluorescent compound, a phosphorescent luminous compound or a material that emits delayed fluorescence. Selection of the organic materials in the organic EL device seriously affects the properties of the device, such as efficiency and durability.
In the organic EL device, the charges injected from both electrodes recombine together in the luminous layer to emit light. In the organic EL device, therefore, what is important is how efficiently to pass the charges of holes and electrons over to the luminous layer. Upon improving the electron injection property, improving the mobility thereof and, therefore, improving the probability of recombination of the holes and the electrons and, further, confining the excitons formed in the luminous layer, it is allowed to attain a high luminous efficiency. Namely, the electron transporting material plays an important role. Therefore, it has been desired to provide an electron transporting material that has a high electron injection property, a high electron mobility, a high hole blocking property and a large durability against the holes.
As for the device life, further, the heat resistance and amorphousness of the material also serve as important factors. The material having low heat resistance is subject to be thermally decomposed even at a low temperature due to the heat generated when the device is driven, and is deteriorated. The material having low amorphousness permits the thin film thereof to be crystallized even in short periods of time and, therefore, the device to be deteriorated. Therefore, the material to be used must have high heat resistance and good amorphousness.
Tris(8-hydroxyquinoline) aluminum (Alq) which is a representative luminous material has also been generally used as an electron transporting material having, however, a hole blocking property which is far from satisfactory.
A method of inserting a hole blocking layer is one of the measures for preventing the holes from partly passing through the luminous layer to improve the probability of recombination of the charges in the luminous layer. As a hole blocking material used for forming the hole blocking layer, there have heretofore been known triazole derivatives (see, for example, a patent document 1), a bathocuproin (BCP), a mixed ligand complex of aluminum [aluminum (III) bis(2-methyl-8-quinolinato)-4-phenyl phenolate (BAlq) and the like.
There has, further, been known a 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (TAZ) as an electron transporting material having excellent hole blocking property (see a patent document 2). The TAZ has a work function of as large as 6.6 eV and a large hole blocking power. Therefore, the TAZ is used as a hole blocking material having an electron transport property, is laminated on the cathode side of a fluorescent luminous layer or a phosphorescent luminous layer prepared by vacuum evaporation or by coating, and is contributing to improving the efficiency of the organic EL devices. Because of its serious problem of low electron transport property, however, the TAZ had to be used in combination with an electron transporting material having a higher electron transport property.
The BCP, on the other hand, has a work function of as large as 6.7 eV and a large hole blocking power but a glass transition temperature (Tg) of as low as 83° C. In the form of a thin film, therefore, the BCP lacks stability and still cannot be said to be sufficiently working as the hole blocking layer.
As described above, either material still lacks stability when it is formed into a film or lacks the function for blocking the holes to a sufficient degree. In order to improve characteristics of the organic EL devices, therefore, it has been desired to provide an organic compound that excels in electron injection/transport performance and in hole blocking power, and features high stability in the form of a thin film.